Upgrade to ESLint 9 for Next.js 16 compatibility

Next.js 16 requires ESLint 9 for proper flat config support. This commit:
- Upgrades eslint from ^8 to ^9
- Migrates from .eslintrc.json to eslint.config.mjs (flat config)
- Updates lint script to use eslint directly (next lint removed in Next.js 16)
- Adds tsconfig.json includes for Next.js 16 dev types
- Fixes layout.tsx to use proper theme CSS link tags

Signed-off-by: Charles de Dreuille <charles.dedreuille@gmail.com>

docs-ui/eslint.config.mjs

@@ -0,0 +1,18 @@
+import { defineConfig, globalIgnores } from 'eslint/config';
+import nextCoreWebVitals from 'eslint-config-next/core-web-vitals';
+
+const eslintConfig = defineConfig([
+  ...nextCoreWebVitals,
+  globalIgnores(['.next/**', 'dist/**', 'node_modules/**']),
+  {
+    rules: {
+      'notice/notice': 'off',
+      'react/forbid-elements': 'off',
+      'jsx-a11y/alt-text': 'off',
+      '@next/next/no-css-tags': 'off',
+      'react-hooks/set-state-in-effect': 'warn',
+    },
+  },
+]);
+
+export default eslintConfig;